not really sure
they may well ask questions about whether you've done jobs where sticking to safety related rules is part of it, or dealing with the public, or dealing with potentially difficult customers, or handled / been accountable for cash (i think you're outside london) they may do tests of basic literacy / numeracy.
the big operators are pretty hot on health + safety in depots, and on drivers following proper procedures (e.g. start of day checks on the bus - you're not expected to be a mechanic, but if you're taking a bus from depot in the morning, you do a basic check of things like all the lights, emergency doors, no obvious issues with the wheels / tyres.)
they may want you to do a test drive in something transit van sort of size (i'm assuming you've not got even provisional PCV entitlement yet, so they can't let you drive a bus on the public road) - if so (and if you've not driven anything that size before) think about how wide a turn you need to make at corners so the back wheel doesn't bump over the kerb, and if you drive a car now, get used to scanning the door / wing mirrors (both sides) not just looking at the mirror in the middle of the windscreen (on a bus, the mirror in the middle of the windscreen, all you can see is inside the bus)
increasingly, bus operators look for people who are likely to be good at the 'people skills' bit of the job and then teach them to drive a bus, rather than look for people with experience driving large vehicles and just hope for the best on the 'people' bit. i don't know exactly what the market is like in your bit of the world, but nationally, there's quite a shortage of bus drivers. (this can mean there's pressure to work overtime and some rest days - legally you can work 13 days out of 14 on local bus, less on coach work.)
there may be a waiting period to get your provisional licence (this involves passing a medical), do the theory test and so on.
they will also ask if you're happy to work shifts - the proportion of early / sunday / late work varies from one place to another, but chances are that you'll at least start on a rota that includes all of these (within reason - you won't have a midnight finish one day and a 5 am start the next) - just how operators put rotas together varies so much from one operator (or even one depot) to another that it's hard for me to be precise. although in general, choice of what rota you go on (at larger depots, different routes or groups of routes may be on different rotas) tends to be based on seniority in the job.
another thing to be aware of - if company is putting you through the PCV test, there will almost certainly be a clause that if you quit within X time (2 years is common) then you will owe them something towards the training (this is broadly because bus operators don't want to provide free training for coach operators.)
www.gov.uk
busdrivers.london
